Structural design of boom section with inverted {37 a{38 {11 frame cross-section

ABSTRACT

A mobile crane has a boom comprising a plurality of telescopic boom sections. Each boom section is fabricated of solid steel plates with the top plate being wider than the bottom plate and the side plates welded therebetween and sloped inwardly. Side plate stiffeners welded between the bottom plate and the side plates run the length of the boom section. This configuration results in a boom section of minimum weight with a bottom plate of relatively increased buckling strength and maximum interior space for boom operating components.

United States Patent Johnston 1 1 Apr. 30, 1974 [541 STRUCTURAL DESIGN OF BOOM SECTION 3,669,281 6/1972 Woodside 212/55 x WITH I T W 3,481,490 12/1969 Eiler 212/55 3,624,979 12/1971 Przybylski 52/1 15 CROSS SECTION 3,719,403 3/1973 Sung 212/55 [75] Inventor: Roger L. Johnston, Cedar Rapids, 3.698.569 10/1972 Lamer 212/55 Iowa Primary Examiner-Henry C. Sutherland [73] Asslgnee' f gz a g Assistant Examiner-Leslie A. Braun 1 Attorney, Agent, or Firm.lames E. Nilles [22] Filed: Sept. 18, 1972 [52] US. Cl 52/115, 52/118, 212/55 [51] Int. Cl. B666 23/04, B660 23/40 [58] Field of Search .,52/114,115,117,118, 52/119; 212/55; 214/141; 182/51 [56] References Cited UNITED STATES PATENTS 3,708,937 1/1973 Sterner 52/115 X 3,719,404 3/1973 Sterner 212/55 X 3,243,052 3/1966 Grove 212/55 3,315,821 4/1967 Grove 212/55 71 ABS TRACT A mobile crane has a boom comprising a plurality of telescopic boom sections. Each boom section is fabricated of solid steel plates with the top plate being wider than the bottom plate and the side plates welded therebetween and sloped inwardly. Side plate stiffeners welded between the bottom plate and the side plates run the length of the boom section. This configuration results in a boom section of minimum weight with a bottom plate of relatively increased buckling strength and maximum interior space for boom operating components.

V 9 Claims, 12 Drawing Figures ATENTEU APR 3 O 1974 SHEET 4 [1F 9 PATHHED 30 I974 SHEET 7 [IF 9 1ATENTEUAPR30 m4 3,807,108

SHEET 9 OF 9 FIG .10

STRUCTURAL DESIGN OF BOOM SECTION WITH INVERTED A FRAME CROSS-SECTION BACKGROUND OF THE INVENTION 1. Field of Use This invention relates generally to booms having a plurality of telescopic boom sections, such as are used on mobile cranes or the like. In particular it relates to booms having boom sections of improved structural design.

2. Description of the Prior Art Present technology requires that mobile cranes and other equipment employing booms having telescopic boom sections be made increasingly larger and stronger so that the booms can be extended to greater lengths and can handle heavier loads. Since, for the practical reasons of cost and reliability, most telescopic boom sections are fabricated from solid sheets or slabs of steel (rather than of a framework of lighter steel members welded or riveted together), any increase in the size of a boom section of conventional design results in a disproportionate increase in weight. This, in turn, results in substantially greater stress and compression loads being imposed on the boom section components. Therefore, stronger and heavier components are needed and this, in turn, further increases the overall boom dimensions and weight. As a result, space and weight considerations become a critical factor in the design of the boom and carriers.

SUMMARY OF THE PRESENT INVENTION In accordance with the present invention there is provided a mobile crane which has a boom comprising a plurality of telescopic boom sections. Each boom section is fabricated of solid steel plates with the top plate being wider than the bottom plate and the side plates welded therebetween and sloped inwardly. Side plate stiffeners welded between the bottom plate and the side plates run the length of the boom section. This crosssectional configuration, which is that of an inverted A- frame or trapezoid, results in a boom section of minimum weight with a bottom plate of relatively increased buckling strength but reduced thickness and maximum interior space for boom operating components. Hydraulic operating cylinders for the boom sections are housed within the boom and each is provided with its own remotely operable electrically controlled main valve. Hydraulic fluid is supplied to the valves by flexible hydraulic fluid hoses disposed between the side plates of adjacent boom sections in looped arrangement and the hoses uncoil or coil in a frictionless manner as the sections are extended or retracted. A boom or boom section in accordance with the present invention is relatively stronger than conventional booms of comparable weight and size, while at the same time providing more usable internal space for a more compact arrangement of boom operating components, such as cylinders, valves and hoses therewithin. Other objects and advantages will hereafter appear.

DRAWINGS FIG. 1 is a side view of a mobile crane having a telescopic boom in accordance with the invention;

FIG. 2 is an enlarged view of the interior of the boom shown in FIG. 1 with the boom sections shown in partially extended position;

FIG. 5 is an enlarged view, with some portions bro- I ken away, of one of the boom sections shown in FIGS. 1, 2 and 3;

FIG. 6 is a view of the boom section shown in FIG. 5, showing a portion of the rear end thereof and a portion in section on line 6-6 of FIG. 5;

FIG. 7 is an enlarged view of the boom shown in FIG. 3', showing a portion of the rear end thereof and a portion in section on line 77 of FIG. 3;

FIG. 8 is an enlarged view, partly in cross section, of one of the hydraulic cylinders shown in FIGS. 2, 3, 7 and 9;

FIG. 9 is a schematic diagram of the hydraulic control system for a boom in accordance with the invention;

FIG. 10 is an enlarged cross section view of an electrically operated hydraulic control valve shown in FIG.

FIG. 11, is a top plan view of taken on line 1 l11 of FIG. 7 showing the hose connections to the control valve for one of the cylinders; and

FIG. 12 is an enlarged cross section view of one of the hydraulic hoses in the boom showing electrical wires for the control valves secured thereto.

DESCRIPTION OF A PREFERRED EMBODIMENT FIG. 1 shows a mobile crane 10 comprising a chassis 11, ground wheels 12, an operators cab 13,'a horizontally rotatable crane upper 14 mounted on the chassis, a telescopic boom 15 in accordance with the invention pivotably mounted on the crane upper, and a pair of boom hoist cylinders 16' (only one visible in FIG. 1) connected between the crane upper and the boom.

' FIGS. 1, 2, 3 and 7 show that boom 15 comprises a plurality of telescopic boom sections, namely: a base section 20, an inner mid section 21, an intermediate mid section 22, an outer mid section 23, a fly section 24, and a manual section 25. As FIGS. 1 and 4 show, the rear end of base section 20 has trunion mounts 26 on opposite sides which receive pins 27' that pivotably mount the base section on crane upper 14. The boom hoist cylinders 16 are located on opposite sides of base section 20 and are pivotably connected by pins 28 to support brackets 29 on crane upper 14. The rod ends of the boom hoist cylinders 16 are pivotably connected by pins 30 to connecting brackets 31 on opposite sides of base section 20. As FIG. 1 shows, the forward end of manual section 25 has a working head 32 thereon which, for example, is provided with a pulley 33.

As FIGS. 2, 3 and 7 show, the boom 15 also comprises a plurality of hydraulic cylinders for extending and retracting the boom sections, namely: an inner mid section cylinder 41, an intermediate mid section cylinder 42, an outer mid section cylinder 43, afly section cylinder 44 and a manual section cylinder 45. The cylinders operate the boom sections bearing the same name. FIG. 8 is an enlarged view, partly in cross section, of outer midsection cylinder 43 and a description thereof will suffice for the cylinders 41, 42, 44 and 45 which are identical thereto except as regards size. Cylinder 43 comprises a hollow tubular housing 50 closed at one end by an end plate 51 within which ahollow tubular cylinder rod 52 is slideably mounted. The inner end of rod 52 is provided with a piston 53 having piston rings 54. Housing 50 and rod 52 cooperate to define a cylinder retract chamber 55 and the end of this chamber is closed off by sealing means 56 secured to housing 50 and having sealing rings 57. Cylinder rod 52 is provided with a cylinder extend chamber 58. Cylinder rod 52 is also provided with an internal passage 59 which is connected by a tube 60 to retract chamber 55. Cylinder rod 52 is further provided with an internal passage 61 which is connected to extend chamber 58. The ports of the passages 59 and 61 are connected by passages 62 and 63, respectively, in an adapter plate 64 on the end of rod 52 to ports 75 and 76, respectively, of an electrically operated cylinder control valve 67 attached to plate 64. Fluid pressure in passage 61 of rod 52 caused by extend operation of valve 67 causes fluid flow into extend chamber 58 and effects extend operation of cylinder 43. Conversely, fluid pressure in passage 59 of rod 52 caused by retract operation of valve 67 causes fluid flow into retract chamber 55 and effects retract operation of cylinder 43.

Housing 50 of cylinder 43 is provided on its exterior with a trunnion 68 by which it is connected to a trunnion mount 69 on outer mid boom section 23. Cylinder rod 52 of cylinder 43 is provided on its exterior with a trunnion 70 by which it is connected to a trunnion 'mount 71 on boom section 22. As FIGS. 2 and 3 show, the trunnion 68 on the housing of each cylinder 41, 42, 43, 44 and 45 is connected to the trunnion mount 69 of that boom section which it operates. The other trunnion 70 on the rod end of the cylinders 41, 42, 43, 44 and 45 is connected to the trunnion mount 71 of the next adjacent boom section.

A hydraulic cylinder such as cylinder 43 offers several advantages over conventional hydraulic actuators or cylinders used in prior art cranes. For example, a conventional cylinder is normally connected and supported at each of its extreme ends (the outer rod end and the base end of the cylinder) to the boom sections associated therewith. Consequently, in extremely large cranes, as the cylinder is operated to extend the boom section, the two connection points of the cylinder move very far apart and the cylinder is subjected to buckling and bending forces which tend to increase the risk of mechanical failure of the cylinder and impose greater wear forces on internal moving components of the cylinder. In cylinder 43, however, the attaching and supporting trunnion 70 of cylinder rod 52 is located near the end of rod 52, whereas the attaching and supporting trunnion 68 of cylinder housing 50 is located near the midpoint of housing 50. Consequently, when cylinder 43 is in retracted or extended condition, the trunnions 68 and 70 are always relatively closer together than the attachment points of a conventional cylinder of comparable size would be. Thus, cylinder 43 is subjected to a reduced load and its buckling strength, especially when fully extended, is substantially increased. This factor is of extreme importance in larger cranes of the type described which have long and heavy boom sections and handle very large loads. I

Referring again to FIG. 8, it is seen that cylinder 43 is provided 'with a conventional holding valve 72'which is mounted on plate 64 and operates to prevent undesired retraction of the extended cylinder until the holding valve is released (opened) in response to retract valve.

The electrically operated main control valve 67 shown in FIG. 10 is an electrohydraulic proportional metering valve comprising a valve housing 73 containing a four-way sliding spool 74, two fluid control ports 75 and 76 which are connected through plate 64 to the passages 59 and 61, respectively, in rod52 of clyinder 43. Valve 67 further comprises an electrical torque motor 80 and a nozzle flapper pilot stage for operating spool 74.

The torque motor 80 includes coils 81, polepieces 82, magnets 83 and an armature 84. The armature 84 is supported for limited movement by a flexure tube 85. The flexure tube 85 also provides a fluid seal between the hydraulic and electromagnetic portions of the valve. i

A flapper 86 attaches to the center of the armature 84 and extends down, inside the flexure tube 85. A nozzle 87 is located on each side of the flapper 86 so that flappermotion varies the nozzle openings. Pressurized hydraulic fluid is supplied to each nozzle through a filter 88 and inlet orifice 89. Differential pressures caused by flapper movement between the nozzles 87 are applied to the ends of the valve spool 74.

The four-way valve spool 74 directs flow from pressure supply chamber 92 to either control port 75 or 76 in an amount proportional to spool displacement. The spool 74 contains flow metering slots 90 in the control lands that are uncovered by spool motion. Spool movement deflects a feedback wire 91 that applies a torque to the armature/flapper. Spool detent springs 92A are provided to center the spool whenever hydraulic driving pressures are absent.

In operation, electrical current in the torque motor coils 81 causes either clockwise or counter-clockwise torque on the armature 84. This torque displaces the flapper 86 between the two nozzles 87. The differential nozzle flow moves the spool 74 to either the right or left. The spool 74 continues to move until the feedback torque counteracts the electromagnetic torque. At this point the armature/flapper is returned to center, so the spool 74 stops and remains displaced until the electrical input changes to a new level. Therefore, valve spool position is proportional to the electrical signal. The actual flow from the valve to the load will depend upon the load pressure.

A valve such as valve 67 is commercially available from Moog, Inc., Controls Division, Proner Airport, East Aurora, New York 14052 and is disclosed in that companys Catalog 602.

FIGS. 5 and 6 show in detail the physical construction and configuration of intermediate mid boom section 22 which will now be described in detail; it being understood that the other boom sections 20, 21, 23, 24 and 25 are similar thereto. Section 22 comprises a top plate 100, a bottom plate 101, a right side plate 102,- and a left side plate 103. Each of these four plates is fabricated of a solid plate or sheet of steel. The upper edges of the side plates 102 and 103 are joined by a continuous weld to the undersurface of the top plate 100. The lower edges of the side plate 102 and 103 are similarly joined by a continuous weld to the upper surface of the bottom plate 101. Side plate stiffeners 105 and 106 are edge-welded between the side plates 102 I and 103, respectively, and the edges of the bottom plate 101. The side plate stiffeners 105 and 106 serve to increase the buckling strength of the side plates 102 and 103, respectively, by about a factor of five in the embodiment shown. The bottom plate 101 is substantially narrower than the top plate 100 (in a ratio of about five units to three units) and the angle a defined between a side plate and the bottom plate is an angle greater than 90, but less, for example, than 120. Each side plate 102 and 103 is wider than the bottom plate 101 in a ratio of about 6 units to 3 units Thus theg eneral cross-sectional configuration of section 22 is that of a trapezoid or an inverted A-frame. In a practical embodiment of the invention, top plate 100 is about 24.6 inches wide, 0.375 inches thick, and 383 inches long. Bottom plate 101 is about 13 inches wide, 0.500 inches thick, and 376 inches long. Each side plate 102 and 103 is about 33 inches wide, 0.150 inches thick, and 380 inches long. Each side plate stiffener 105 and 106 runs the length of section 22 and is about 6.375 inches wide and 0.250 inches thick.

It has been discovered through testing that boom section 22 affords the following advantages. First, the internal height of boom section 22 is increased without an increase in other cross section properties, such as additional weight, thereby allowing more internal space for accommodating components such as the hydraulic cylinders. Second, the stiffener plates 105 and 106 change the end condition of the side plates 102 and 103 from simply supported to semi-fixed and thereby allow thinner side plates to be used in view of buckling considerations. Third, the relatively narrower bottom plate 101 allows a thinner bottom plate to be used in view of buckling considerations. Fourth, as FIG. 4 shows, the relative narrowness of the bottom of a boom section such as 22 (particularly boom base section 20) allows the storage position of boom to be lower between the boom hoist cylinders 16 thereby affording lower overall height without increasing boom section width and without reducing the moment arm of the boom hoist cylinders. Fifth, the width of the top plate 100 of boom section 22 can be varied for desired side strength purposes without affecting any of the above-listed advantages.

Referring again to FIGS. 5 and 6, boom section 22 further comprises a slide pad support plate 110 welded to the side plates 102 and 103 at the top rear end of the boom section. A pair of external slide plates 111 and 112 are welded to plate 110 and serve as bearing surfaces for the undersurface of the top plate 100 of the boom section 21 into which boom section 22 telescopes. Support plate 110 is braced by welded external braces 113 and 114 and by a welded horizontal inner brace 115 to which spaced apart vertical braces 116 are welded.

Rear stiffener plates such as 117 are provided at the rear end of boom section 22 and are welded to support plate 110 and an associated side plate 102 or 103.

A cylinder attachment assembly 71 and a trunnion mount 69, both hereinbefore referred to in connection with the description of cylinder 43, are provided at the outer rear end of boom section 22.

Boom section supports, such as support 120, are welded on the inner surface of the side plates 102 and 103 near the rear end of boom section 22. Each support 120 is rigidified and stabilized by a brace 121.

A pair of internal slide pads, such as pad 122, are mounted on a support 123, which is welded to the inner surfaces of the side plates 102 and 103 near the forward end of boom section 22. Support 123 is rigidified and strengthened by a welded horizontal brace 124 and welded vertical braces 125.

Polyethylene slide pads, such as 130, are mounted on steel back-up plates, such as 131, and extend through openings, such as 132, in the side plates 102 and 103 into the interior of boom section 22 where they frictionally engage the sides of boom section 23 which telescopes into boom section 22. Slide pads 135, similar to project outwardly on the sides of the forward end of boom section 22 for engagement with boom section 21 into which boom section 22 telescopes.

Referring now to FIGS. 2, 7 and 11, it is seen that the manual section cylinder 45, the fly section cylinder 44, and the outer mid section cylinder 43 fit within manual boom section 25 when boom 15 is in retracted condition. Intermediate mid section cylinder 42 and inner mid section cylinder 41 fit within intermediate mid boom section 22 and inner mid boom section 21, respectively. The main control valves 67A, 67B, 67 67C and 67D for the cylinders 41, 42, 43, 44 and 45, respectively, are supplied with hydraulic operating fluid from a pressurized fluid source, such as pumps 136 and 137 shown in FIG. 9, which is understood to be located on crane 10.

FIG. 11 is a top plan view of valve 67 on outer mid cylinder 43 and shows that hydraulic fluid T-fittings 140 and 141 are provided on opposite sides thereof. It is to be understood, for example, that fittings 140 and 141 are located on the extend side and the retract side, respectively, of valve 67. Furthermore, valve 67 is provided internally with a pressure compensated flow control device 142 into which T-fitting 140 discharges. The fittings 140 and 141 on valve 67 are connected by flexible hoses 149B and 150B, respectively, to the pumps 136 and 137 and to a fluid reservoir 147. Thus, hose 149B is for fluid supply and hose 1503 for fluid return. The fittings 140 and 141 are also connected to fluid supply and fluid return hoses 149C and 150C, respectively, which are connected to the control valve 67C for the next cylinder 44. As FIG. 9 shows, the main control valves 67A, 67B, 67C and 67D for the other cylinders are each provided with fluid supply and return T-fittings designated respectively: 140A and 141A; 1408 and 1418; 140C and 141C; 140D and 141D. These fittings are interconnected by flexible fluid supply and return hoses designated, respecively: 149A and 150A; 1498 and 150B; 149C and 150C; 149D and 150D. Thus the main control valves are connected in parallel arrangement to the fluid'supply and fluid return systems. As FIG. 9 also shows, the valves 67A, 67B, 67C, and 67D are provided with pressure compensated flow control devices 142A, 142B, 142C and 142D, respectively.

The hydraulic control system shown in FIG. 9 also includes a pressure responsive valve 156 for automatically connecting pump 137 to the system if system pressure indicates that more than one cylinder is being extended and additional fluid is required. The pumps 136 and 137 are understood to be engine-driven and operating continuously when crane 10 is in use. The hydraulic control system also includes a pressure compensated flow control valve 157, a pressure switch 158, a relief valve 159, and solenoid controlled pressure relief valves 160 and 161.

As FIGS. 2 and 7 show, the flexible fluid supply and fluid return hoses such as 149C and 150C are physically located between the side plates of adjacent boom sections in a looped or coiled arrangement which enables the hoses ,to pay out or coil back upon themselves as the boom sections are extended or retracted, respectively. As FIG. 12 shows, the lower portion of each hose loop is supported or rests in support clips 155 which are secured at intervals on the outer surfaces of the boom side plates. It is to be noted that, although each hose flexes at the curved portion as the boom 15 is operated, the straight upper and lower segments of the hose do not slide on, rub or frictionally engage the sides of the adjacent boom sections but are stationary with respect thereto. Consequently, the hoses are not subjected to damaging abrasion or wear from this source but only to flexing action.

As hereinbefore expalined, the main control valves 67, 67B, 67C and 67D are electrically operated in response to electric signals supplied to the torque motor thereon, such as torque motor 80 hereinbefore described in connection with valve 67. As FIG. 9 shows, each of the four electrically controlled main valves is connected by electrical conductors, generally designated 165, which extend from an electrical controller 166 on crane 10 to the valves. In practice only two conductors 165 are necessary for each main valve, although additional back-up conductors could be provided for each valve as a safety feature. Furthermore, it isto be understood that controller 166 is designed to enable actuation of each valve separately to allow any single boom section to be operated individually or to enable actuation of all valves simultaneously to allow all boom sections to be operated simultaneously and proportionately.

As FIG. 10 shows, the electrical conductors 165 for the valves are connected to a terminal block 167 provided on each valve. Between each valve, as FIG. 12 shows, the conductors 165 are twisted around a hydraulic hose, such as hose 149C, and secured in place thereon by means of a flexible heat-shrunk plastic sleeve or tube 168. Preferably, the conductors 165 take the form of braided insulated wires. This arrangement is advantageous in that the wires are securely held in place, not liable to damage as the boom sections move, and like the hoses, are not subject to frictional wear but only flexure.

The boom extension cylinders 45, 44,43, 42 and 41 are sized progressively larger from top to bottom so that, although the working pressure required is least at the uppermost cylinder 45, the combined effect of required working pressure, induced load pressure and line pressure drops is such that pressure requirements as measured at the pump 136 or 137, or even at the boom base cylinder 41, are approximately equal for any given load, regardless of which cylinder or combination of cylinders are in use.

It is important to note, as FIG. 9 shows, that there is only one pressure compensated valve 67A, with auxiliary valves, such as 157, for providing unloading and main relief capabilities near the boom base, for directly controlling inner mid section cylinder 41. The other four valves 67B, 67, 67C and 67D are remote controlled electro hydraulic proportional metering valves mounted on cylinders in the boom. In practice, valve 67A, valve 157, valve 159 and all necessary interconnecting tubing are housed in a common valve housing (not shown).

The pressure compensating valve 157 functions by constantly measuring the difference between pump and cylinder pressure and dumping or unloading the excess pump flow as required to maintain the desirable differential. This would normally be impossible in conventional hydraulic control circuits for cranes when the most remote cylinder is 125 feet away, but if the system balance is such as is possible in the present invention so that the nearest cylinder pressure reading is the same as the furthest, then it is only necessary to measure and respond to the nearest pressure condition, and the unloading function of valve 157 serves the furthest cylinder equally as well as the nearest cylinder.

RESUME A mobile crane 10 has a boom 15 comprising a plurality of telescopic boom sections 20, 21, 22, 23, 24 and 25. Each boom section is fabricated of solid steel plates with the top plate being wider than the bottom plate 101 and the side plates 102 and 103 welded therebetween and sloped inwardly. Side plate stiffeners 105 and 106 welded between the bottom plate 101 and the side plates 102 and 103, respectively, run the length of the boom section. This configuration results in a boom section-of minimum weight with a bottom plate 101 of relatively increased buckling strength and maximum interior space for boom operating components. Hydraulic operating cylinders 41, 42, 43, 44 and 45 for the boom sections 21, 22, 23, 24 and 25, respectively, are housed within the boom and each is provided with its own remotely operable electrically controlled main valve such as valve 67. Hydraulic fluid is supplied to and returnd from the valves by flexible hydraulic fluid hoses such as hoses 149C and 150C disposed between the side plates of adjacent boom sections and the hoses uncoil or coil in a frictionless manner as the boom sections are extended or retracted. Electric control wires, such as 165, for operating the electric valves are wrapped about the hoses and held in place thereon by heat-shrunk plastic tubing 168.

Boom 15 in accordance with the present invention is relatively stronger than conventional booms of comparable weight and size, while at the same time enabling a more compact arrangement of boom operating'components, such as cylinders and hoses, therewithin. F urthermore, the hose means for supplying hydraulic fluid to the cylinders within the boom are simpler and more compact than known hose arrangements.

I claim:

1. In combination in a crane: a support; a boom having at least one hollow boom section mounted for vertical pivotal movement on said support; and boom hoist means for effecting vertical pivotal movement of said boom, said boom section comprising:

a top plate defining an upper side of said boom section, which upper side is the side furthest away from said support,

a bottom plate narrower than said top plate and defining a lower side of said boom section, which lower side is the side closest to said support,

and two side plates connected between the lower surface of the top plate and the upper surface of the bottom plate, each side plate being wider than said bottom plate,

and each side plate defining an angle between its inner surface and the upper surface of said bottom plate which is greater than 90 but less than 120.

2. A combination according to claim 1 including two elongated side plate stiffeners, each extending along the length of said boom section and connected between the outer surface of one side plate and said bottom plate.

3. A combination according to claim 2 wherein each side plate stiffener connects to its respective side plate along a line which is approximately 1/6 of the width of the side plate above said bottom plate to thereby increase the buckling strength of said side plate by a factor of approximately five.

4. A combination according to claim 3 and wherein said one boom section is fabricated of steel plate, wherein said two side plates are welded between the lower surface of said top plate and the upper surface of said bottom plate, and wherein each of said side plate stiffeners is welded to the outer surface of its associated side plate and to an edge of said bottom plate.

5. A combination according to claim 4 wherein said one boom section is a base section and wherein said boom hoist means comprises a pair of boom hoist cylinders on opposite sides of said boom and connected between said support and said boom base section and spaced from each other a distance greater than the width of the bottom side of the boom but less than the width of the top sideof said boom whereby said boom base section nests therebetween when in lowered position.

6. In combination in a crane:

a support;

a telescopic boom comprising a hollow base section and at least one other hollow section mounted for telescopic movement with respect to said base sec-' tron;

means for pivotally connecting said base section to said support to enable vertical pivotal movement of said boom;

means for extending and retracting said other boom section with respect to said base section;

boom hoist means connected to and between said support and said base section for effecting vertical pivotal movement of said boom to raise and lower said boom;

external slide pads located at the upper rear end of said other section; and

internal slide pads located inside of and near the forward lower end of said base section;

each of said boom sections comprising:

a top plate defining an upper side of said one boom section, which upper side is the side fiirthest away from said support,

a bottom plate narrower than said top plate and defining a lower side of said one boom section, which lower side is the side closest to said support,

two side plates connected between the lower surface nected between the outer surface of one side plate and said bottom plate.

7. A combination according to claim 6 including support means for said external slide pads comprising: a slide pad support plate connected to the top edges of said side plates, an inner brace below and spaced from said slide pad support plate and connected to the inner surfaces of said side plates, and a plurality of vertical braces secured to and between said slide pad support plate and said inner brace.

8. A combination according to claim 7 including support means for said internal slide pads comprising:

a support connected to the inner surfaces of said side plates, a horizontal brace below and spaced from said support and connected to the inner surfaces of said side plates, and a plurality of vertical braces secured to and between said support and said horizontal brace.

9. In combination in a mobile crane:

a support;

a telescopic boom comprising a hollow base section and a plurality of other hollow sections mounted for telescopic movement with respect to said base section; I

means'for pivotally connecting said base section to said support to enable vertical pivotal movement of said boom;

means for extending and retracting said other boom section with respect to said base section;

external slide pads located at the upper rear end of each of said other sections; and

internal slide pads located inside of and near the forward lower end of said base section and at least some of said other sections;

each of said boom sections being fabricated of steel plate and comprising:

a top plate defining an upper side of said one boom section, which upper side is the side furthest away from said support,

a bottom plate narrower than said top plate and defining a lower side of said one boom section, which lower side is the side closest to said support,

two side plates welded between the lower surface of the top plate and the upper surface of the bottom plate, each side plate being wider than said bottom plate, and each side plate defining an angle between its inner surface and the upper surface of said bottom plate which is greater than but less than two elongated side plate stiffeners, each extending along the length of said boom section and welded between the outer surface of one side plate and said bottom plate;

and boom hoist means connected to and between said support and said base section for effecting vertical pivotal movement of said boom to raise and lower said boom, said boom hoist means comprising a pair of boom hoist cylinders on opposite sides of said boom and connected between said support and said boom base section and spaced from each other a distance greater than the width of the bottom side of the boom base section but less than the width of the top side of said boom base section whereby said boom base section nests therebetween when in lowered position. 

1. In combination in a crane: a support; a boom having at least one hollow boom section mounted for vertical pivotal movement on said support; and boom hoist means for effecting vertical pivotal movement of said boom, said boom section comprising: a top plate defining an upper side of said boom section, which upper side is the side furthest away from said support, a bottom plate narrower than said top plate and defining a lower side of said boom section, which lower side is the side closest to said support, and two side plates connected between the lower surface of the top plate and the upper surface of the bottom plate, each side plate being wider than said bottom plate, and each side plate defining an angle between its inner surface and the upper surface of said bottom plate which is greater than 90* but less than 120*.
 2. A combination according to claim 1 including two elongated side plate stiffeners, each extending along the length of said boom section and connected between the outer surface of one side plate and said bottom plate.
 3. A combination according to claim 2 wherein each side plate stiffener connects to its respective side plate along a line which is approximately 1/6 of the width of the side plate above said bottom plate to thereby increase the buckling strength of said side plate by a factor of approximately five.
 4. A combination according to claim 3 and wherein said one boom section is fabricated of steel plate, wherein said two side plates are welded between the lower surface of said top plate and the upper surface of said bottom plate, and wherein each of said side plate stiffeners is welded to the outer surface of its associated side plate and to an edge of said bottom plate.
 5. A combination according to claim 4 wherein said one boom section is a base section and wherein said boom hoist means comprises a pair of boom hoist cylinders on opposite sides of said boom and connected between said support and said boom base section and spaced from each other a distance greater than the width of the bottom side of the boom but less than the width of the top side of said boom whereby said boom base section nests therebetween when in lowered position.
 6. In combination in a crane: a support; a telescopic boom comprising a hollow base section and at least one other hollow section mounted for telescopic movement with respect to said base section; means for pivotally connecting said base section to said support to enable vertical pivotal movement of said boom; means for extending and retracting said other boom section with respect to said base section; boom hoist means connected to and between said support and said base section for effecting vertical pivotal movement of said boom to raise and lower said boom; external slide pads located at the upper rear end of said other section; and internal slide pads located inside of and near the forward lower end of said base section; each of said boom sections comprising: a top plate defining an upper side of said one boom section, which upper side is the side furthest away from said support, a bottom plate narrower than said top plate and defining a lower side of said one boom section, which lower side is the side closest to said support, two side plates connected between the lower surface of the top plate and the upper surface of the bottom plate, each side plate being wider than said bottom plate, and each side plate defining an angle between its inner surface and the upper surface of said bottom plate which is greater than 90* but less than 120*; and two elongated side plate stiffeners, each extending along the length of said boom section and connected between the outer surface of one side plate and said bottom plate.
 7. A combination according to claim 6 including support means for said external slide pads comprising: a slide pad support plate connected to the top edges of said side plates, an inner brace below and spaced from said slide pad support plate and connected to the inner surfaces of said side plates, and a plurality of vertical braces secured to and between said slide pad support plate and said inner brace.
 8. A combination according to claim 7 including support means for said internal slide pads comprising: a suPport connected to the inner surfaces of said side plates, a horizontal brace below and spaced from said support and connected to the inner surfaces of said side plates, and a plurality of vertical braces secured to and between said support and said horizontal brace.
 9. In combination in a mobile crane: a support; a telescopic boom comprising a hollow base section and a plurality of other hollow sections mounted for telescopic movement with respect to said base section; means for pivotally connecting said base section to said support to enable vertical pivotal movement of said boom; means for extending and retracting said other boom section with respect to said base section; external slide pads located at the upper rear end of each of said other sections; and internal slide pads located inside of and near the forward lower end of said base section and at least some of said other sections; each of said boom sections being fabricated of steel plate and comprising: a top plate defining an upper side of said one boom section, which upper side is the side furthest away from said support, a bottom plate narrower than said top plate and defining a lower side of said one boom section, which lower side is the side closest to said support, two side plates welded between the lower surface of the top plate and the upper surface of the bottom plate, each side plate being wider than said bottom plate, and each side plate defining an angle between its inner surface and the upper surface of said bottom plate which is greater than 90* but less than 120*; two elongated side plate stiffeners, each extending along the length of said boom section and welded between the outer surface of one side plate and said bottom plate; and boom hoist means connected to and between said support and said base section for effecting vertical pivotal movement of said boom to raise and lower said boom, said boom hoist means comprising a pair of boom hoist cylinders on opposite sides of said boom and connected between said support and said boom base section and spaced from each other a distance greater than the width of the bottom side of the boom base section but less than the width of the top side of said boom base section whereby said boom base section nests therebetween when in lowered position. 